In recent years such an image forming device of toner system as exemplified by a laser printer has been widely used to enable a clear image with excellent resolution to be produced.
Here, a prior art image forming device of quadrature transfer system will be first explained.
FIG. 9 is a diagrammatic sketch of a prior art image forming device of quadrature transfer system, FIG. 10 is a perspective view of the foregoing image forming device and FIG. 11 is a diagram to show the transfer state of an opposed transfer roller of the foregoing image forming device. In FIG. 9, FIG. 10 and FIG. 11, the reference symbol 1 is a photosensitive belt, the reference symbol 2 is a charging unit, the reference symbol 3 is an image developer, the reference symbol 3a is toner, the reference symbol 3b is an image development roller, the reference symbol 3c is a toner image, the reference symbol 4 is a photosensitive belt cleaner, the reference symbol 5 is a discharging unit, the reference symbol 6 is a light exposure unit, the reference symbol 7 is a print sheet, the reference symbol 8 is an opposed transfer roller, the reference symbols 9a to 9c are photosensitive rollers, the reference symbol 10 is a transfer plate and the reference symbol 11 is a print sheet transporting roller.
As shown in FIG. 9, a closed loop belt like shape photosensitive belt 1, the surface of which is coated with an organic photoconductive material, is supported by photosensitive rollers 9a to 9c and an opposed transfer roller 8 and also made movable rotationally in the direction of an arrow A. A photosensitive material cleaner 4, a discharging unit 5, a charging unit 2, a light exposure unit 6 and an image developer 3 are arranged in this order along the direction of a rotational movement of the photosensitive belt 1, and further toner 3a is contained inside of the image developer 3. In addition, a print sheet 7 is supported by a transfer plate 10 and transported in the direction C perpendicular to the rotational direction A of the photosensitive belt 1 by a print sheet transporting roller 11 as shown in FIG. 10.
An explanation will be made on how a prior art image forming device of quadrature transfer system as structured above operates. Upon receiving directions for an image forming operation from a controller of the image forming device such as a CPU and the like, a photosensitive roller driving means such as a motor and the like make the photosensitive rollers 9a to 9c rotate, thereby starting the rotation of the photosensitive belt 1 in the direction of the arrow A. First, the photosensitive belt 1 has the surface thereof charged by the charging unit 2 and then an electrostatic latent image is formed on the charged surface by laser light 6a irradiated from the light exposure unit 6 according to the content of an image to be formed.
When the area of the photosensitive belt 1 where the electrostatic latent image has been formed arrives at the position of the image developer 3, an image development roller 3b installed in the image developer 3 is rotated by an image development roller driving means such as a motor and the like and at the same time brought into contact with the photosensitive belt 1. Accordingly, while the photosensitive belt 1 is sandwiched and moved between the image development roller 3b and the photosensitive roller 9b, the electrostatic latent image is developed by the toner 3a attached to the surface of the image development roller 3b and a toner image 3c is formed on the photosensitive belt 1.
After the toner image 3c corresponding to a single sheet of the print sheet in size has been formed on the photosensitive belt 1 in the foregoing step of operation, the toner image 3c is transferred from the photosensitive belt 1 to the print sheet 7.
Before a transfer of the toner image 3c takes place, the rotation of the photosensitive rollers 9a to 9c is suspended with a resulting stop of the rotational movement of the photosensitive belt 1 when a tip p 1 (FIG. 9) of the toner image 3c formed on the photosensitive belt 1 arrives at almost the same position as an end p 2 of the image forming area of the print sheet 7.
Then, the print sheet transporting roller 11 is rotated by a print sheet transporting roller driving means such as a motor and the like and the print sheet 7 is transported on the transfer plate 10 in the direction C (print sheet transporting direction) perpendicular to the direction A of the rotational movement of the photosensitive belt 1 to a specified position. Upon completion of transporting of the print sheet 7, the opposed transfer roller 8 is moved horizontally along the surface of the print sheet 7 in the direction of an arrow B as shown in FIG. 10 and FIG. 11 from a transfer starting position p 3 to a transfer ending position p 4 (FIG. 11). While this horizontal travel of the opposed transfer roller 8 is taking place, the toner image 3c on the photosensitive belt 1 is transferred on the print sheet 7 through consecutive steps of having the photosensitive belt 1 pressed against the print sheet 7 by the opposed transfer roller 8.
When the transferring of the toner image 3c is finished, the print sheet transporting roller 11 starts to rotate again and the print sheet 7, on which an image is formed, is rolled out. When the rolling out of the print sheet 7 is finished, the photosensitive belt 1 starts to move again, the area, where the toner image 3c was transferred on the photosensitive belt 1, gets rid of the toner that remained on the photosensitive belt 1 by means of the photosensitive belt cleaner 4 and then the electric charge that remained on the photosensitive belt 1 is made to disappear by means of the discharging unit 5.
Accordingly, a given image such as lettering, graphics and the like is formed on the single sheet of the print sheet 7, and the same image forming process as above is repeated in succession as needed.
Next, an explanation will be made on a prior art image forming device of parallel transfer system.
FIG. 12 is a diagrammatic sketch of a prior art image forming device of parallel transfer system, FIG. 13 is a perspective view of the image forming device of FIG. 12 and FIG. 14 is a diagram to show the transfer state of an opposed transfer roller of the image forming device of FIG. 12. Here, an explanation will be made only on the part where the foregoing image forming device differs from the previously described image forming device of quadrature transfer system.
In FIG. 12 and FIG. 13, a photosensitive belt 1 is wound around photosensitive rollers 9a, 9b and 9c that are rotated by means of a motor and the like so as to make the photosensitive belt 1 movable in the direction indicated by an arrow A. The movement direction of this photosensitive belt 1 is the same as the feeding direction of a print sheet 7 that is supplied onto a transfer plate 10 by means of a print sheet transporting roller 11, and the print sheet 7 is supported by the transfer plate 10 and transported in the same direction as the photosensitive belt 1 is moved by means of the print sheet transporting roller 11 as shown in FIG. 13.
How a toner image 3c formed on the photosensitive belt 1 is transferred onto the print sheet 7 in the prior art image forming device is as described below.
The print sheet transporting roller 11 is rotated by a driving means such as a motor and the like, thereby transporting the print sheet 7 on the surface of the transfer plate 10 in the same direction as the moving direction of the photosensitive belt 1 to a specified position. Upon finishing the transporting of the print sheet 7, an opposed transfer roller 8 is moved horizontally on the surface of the print sheet 7 in the direction indicated by an arrow B in FIG. 14 from a transfer starting position p 3 to a transfer ending position p 4. While the horizontal travel of this opposed transfer roller 8 is taking place, the toner image 3c on the photosensitive belt 1 is transferred onto the print sheet 7 through consecutive steps of having the photosensitive belt 1 pressed against the print sheet 7 by the opposed transfer roller 8, thereby forming a printed image 3d on the print sheet 7 as shown in FIG. 13. Further, when the opposed transfer roller 8 has arrived at the transfer ending position p 4, the opposed transfer roller 8 starts to move in the reversed direction and returns to the transfer starting position p 3.
However, with the foregoing prior art image forming device, the opposed transfer roller 8 travels the maximum width between p 3 and p 4 even if a small sized print sheet is used, thereby causing the extra amount of toner left on the photosensitive belt 1 at the place, where no pressing on the print sheet takes place, to get shifted toward the transfer plate 10. Thereafter, when a large sized print sheet 7 is used, the toner shifted onto the transfer plate 10 is attached to the back side surface of the print sheet 7 to present a problem of form a smear (back smear). Further, regardless of the size of the image to be printed, the traveling distance of the opposed transfer roller 8 remains the same and so the time required for image transferring also remains the same, resulting in a problem of reduced efficiency.